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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022 | Dec. 31, 2022
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | $ 1,488,622 |  | $ 1,488,622 |  | $ 1,420,641
Total Unrealized Gains |  |  | 328,510 |  | 337,994
Total Unrealized Losses |  |  | (74,406) |  | (69,091)
Amortized cost | 1,079,000 |  | 1,079,000 |  | 927,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 34,897 | $ 788 | 71,338 | $ 8,093 |
Realized losses | (12,115) | (6,908) | (29,001) | (26,582) |
Proceeds from the sale of securities | 578,140 | 218,817 | 1,145,668 | 910,487 |
Nuclear Decommissioning Trusts |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 1,120,463 |  | 1,120,463 |  | 1,073,410
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 34,897 | 788 | 71,338 | 8,093 |
Realized losses | (11,568) | (6,908) | (28,454) | (26,582) |
Proceeds from the sale of securities | 487,324 | 153,573 | 922,270 | 783,232 |
Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 368,159 |  | 368,159 |  | 347,231
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 0 | 0 | 0 | 0 |
Realized losses | (547) | 0 | (547) | 0 |
Proceeds from the sale of securities | 90,816 | $ 65,244 | 223,398 | $ 127,255 |
Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 499,316 |  | 499,316 |  | 554,214
Total Unrealized Gains |  |  | 327,887 |  | 334,817
Total Unrealized Losses |  |  | (23) |  | (267)
Equity securities | Nuclear Decommissioning Trusts |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 444,063 |  | 444,063 |  | 487,240
Equity securities | Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 55,253 |  | 55,253 |  | 66,974
Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 1,005,134 |  | 1,005,134 |  | 861,637
Total Unrealized Gains |  |  | 623 |  | 3,177
Total Unrealized Losses |  |  | (74,383) |  | (68,795)
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 96,094 |  | 96,094 |  |
1 year – 5 years | 421,465 |  | 421,465 |  |
5 years – 10 years | 166,450 |  | 166,450 |  |
Greater than 10 years | 321,125 |  | 321,125 |  |
Total | 1,005,134 |  | 1,005,134 |  |
Available for sale-fixed income securities | Nuclear Decommissioning Trusts |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 693,510 |  | 693,510 |  | 582,343
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 18,012 |  | 18,012 |  |
1 year – 5 years | 194,625 |  | 194,625 |  |
5 years – 10 years | 163,662 |  | 163,662 |  |
Greater than 10 years | 317,211 |  | 317,211 |  |
Total | 693,510 |  | 693,510 |  |
Available for sale-fixed income securities | Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 311,624 |  | 311,624 |  | 279,294
Available for sale-fixed income securities | Coal Reclamation Escrow Account |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 59,191 |  | 59,191 |  |
1 year – 5 years | 42,158 |  | 42,158 |  |
5 years – 10 years | 0 |  | 0 |  |
Greater than 10 years | 3,914 |  | 3,914 |  |
Total | 105,263 |  | 105,263 |  |
Available for sale-fixed income securities | Active Union Employee Medical Account |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 18,891 |  | 18,891 |  |
1 year – 5 years | 184,682 |  | 184,682 |  |
5 years – 10 years | 2,788 |  | 2,788 |  |
Greater than 10 years | 0 |  | 0 |  |
Total | 206,361 |  | 206,361 |  |
Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | (15,828) |  | (15,828) |  | 4,790
Total Unrealized Gains |  |  | 0 |  | 0
Total Unrealized Losses |  |  | 0 |  | (29)
Other | Nuclear Decommissioning Trusts |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | (17,110) |  | (17,110) |  | 3,827
Other | Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | $ 1,282 |  | $ 1,282 |  | $ 963

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